WO2021143151A1

WO2021143151A1 - Catalyst, preparation method therefor, and process for electrocatalytic decomposition of water into hydrogen

Info

Publication number: WO2021143151A1
Application number: PCT/CN2020/112475
Authority: WO
Inventors: 郑南峰; 李智森; 刘圣杰
Original assignee: 厦门大学
Priority date: 2020-01-16
Filing date: 2020-08-31
Publication date: 2021-07-22
Also published as: CN111151255B; CN111151255A

Abstract

Disclosed are a catalyst, a preparation method therefor, and a process for the electrocatalytic decomposition of water into hydrogen. A base of a catalyst is a copper material, wherein the surface of the copper material is coated with metal nickel of a cubic phase structure, the surface of the nickel metal with a cubic phase structure is coated with nickel metal of a hexagonal structure, and nickel oxide or nickel hydroxide is provided on the surface of the nickel metal with a hexagonal structure, wherein the thickness of the nickel metal with a cubic phase structure is 10 to 200 nm; the thickness of the nickel metal with a hexagonal structure is 3 to 14 nm; and the thickness of the nickel oxide or nickel hydroxide is 1 to 5 nm. The catalyst has the characteristics of a high efficiency, a good stability, a low cost, and a simple preparation method in the process for the electrocatalytic decomposition of water into hydrogen, and has great potential application value.

Description

Catalyst, preparation method and electrocatalytic decomposition of water to produce hydrogen process

Technical field

The invention belongs to the technical field of catalysts, and relates to a catalyst, a preparation method and a process for electrocatalytically decomposing water to produce hydrogen.

Background technique

Hydrogen energy is regarded as the clean energy with the most development potential in the 21st century. Commonly used hydrogen production technologies include hydrogen production from coal, hydrogen production from natural gas and petroleum, industrial by-product hydrogen, hydrogen production from electrolysis of water, etc. The hydrogen production technology from electrolysis of water mainly includes alkaline water electrolyzer (AE), proton exchange membrane water electrolysis Cells (PEM) and Solid Oxide Water Electrolysis Cells (SOE). AE technology is the most mature, and the production cost is relatively low, and it has entered the practical stage. However, the main problem that restricts the promotion of AE technology is cost. Costs include electricity prices and catalyst costs. Since electricity prices are relatively fixed, reducing the cost of catalysts becomes a must. There are three main ways to reduce the cost of the catalyst: one is to reduce the cost of the catalyst itself, the other is to increase the catalytic activity of the catalyst, and the third is to increase the use time of the catalyst.

At present, the catalysts used in AE technology are mainly precious metal catalysts, which have the problems of small reserves, high costs, and difficulty in large-scale application. Due to its abundant reserves, low price, and non-precious metal catalysts with the theoretical hydrogen evolution activity closest to precious metals, metallic nickel has rich application prospects for industrial production.

However, non-precious metal catalysts have encountered several problems in practical applications: (1) Highly active transition metal catalysts are easily oxidized in air, thereby losing their catalytic activity; (2) It is difficult for ordinary catalysts to work in the actual hydrogen production environment. Work stably for a long time; (3) Lack of simple and cheap synthetic methods to prepare highly efficient and stable catalysts.

Summary of the invention

The purpose of the present invention is to provide a metal nickel-based multilayer coating structure with an original catalyst, which has good structural stability, no precious metals, and low cost.

Another object of the present invention is to provide a catalyst preparation method, which has simple raw materials, low price, simple preparation method, low cost, and large-scale production.

Another object of the present invention is to provide a process for producing hydrogen by electrocatalytic decomposition of water, which adopts the catalyst of the present invention and has the characteristics of high catalytic activity and good stability.

The present invention adopts the following technical solutions,

A catalyst, the substrate of the catalyst is a copper material, the surface of the copper material is covered with a cubic phase structure of nickel, the surface of the cubic phase structure of the nickel is covered with a hexagonal phase structure of nickel, and the surface of the hexagonal phase structure of the nickel Nickel oxide or nickel hydroxide is provided; the thickness of the cubic phase structure metallic nickel is 10 to 200 nm, the thickness of the hexagonal phase structure metallic nickel is 3 to 14 nm, and the thickness of the nickel oxide or nickel hydroxide is 1 to 5nm.

Preferably, the thickness of the cubic phase structure metallic nickel is 30-180 nm. More preferably, the thickness of the cubic phase structure metallic nickel is 50-130 nm.

Preferably, the thickness of the hexagonal phase structure metallic nickel is 4-12 nm. More preferably, the thickness of the hexagonal phase structure metallic nickel is 5-10 nm.

Preferably, the thickness of the nickel oxide or nickel hydroxide is 2 to 4 nm.

Preferably, the copper material is selected from at least one of foamed copper, copper powder, copper mesh and copper foil.

A method for preparing the catalyst according to any one of the above embodiments, comprising the following steps:

(1) The copper material is ultrasonically degreased and degreasing in acetone or absolute ethanol, washed with ultrapure water, placed in hydrochloric acid to ultrasonically remove surface oxides, and then cleaned with ultrapure water to obtain a pretreated copper material;

(2) In parts by weight, 1 part of sodium formate is added to 4-20 parts of water to obtain a sodium formate solution;

(3) In terms of parts by weight, 1 part of nickel source is added to 15-60 parts of N,N-dimethylformamide (DMF) to obtain a nickel source solution;

(4) Add the pretreated copper material obtained in step (1), the sodium formate solution obtained in step (2), and the nickel source solution obtained in step (3) into the reaction kettle, seal, and react at 150-180°C After 12-24 hours, cool, take out the copper material, clean, vacuum dry, perform electrochemical hydrogen evolution polarization curve test, take it out, and place it in an oxygen atmosphere at room temperature for 5-20 hours to obtain the catalyst.

Preferably, the nickel source in step (3) is selected from at least one of nickel chloride, nickel sulfate, nickel nitrate and nickel acetylacetonate.

Preferably, in step (4), after the sodium formate solution and the nickel solution are added to the reaction kettle, the liquid surface is immersed in the copper material.

Preferably, after copper material, sodium formate solution and nickel source solution are added to the reaction kettle in step (4), the reaction filling ratio is 45-75%.

Preferably, after adding copper material, sodium formate solution and nickel source solution to the reaction kettle in step (4), before sealing, the reaction kettle is placed in an ultrasonic generator for ultrasonic vibration for 5-15 minutes.

Preferably, the oxygen atmosphere in step (4) is selected from oxygen or a mixed gas containing oxygen.

More preferably, the volume fraction of oxygen in the mixed gas is not less than 5%.

Further preferably, the mixed gas is air.

An electrocatalytic decomposition of aquatic hydrogen production process, the catalyst of the process uses the catalyst described in any one of the above embodiments.

The beneficial effects of the present invention:

(1) The catalyst of the present invention has many catalytic active sites at its surface interface, and the exposed surface of the surface has a hexagonal phase structure, which forms a crystalline heterojunction with the cubic nickel in the inner layer; the hexagonal phase structure on the surface further passes through the surface Modification of oxides and hydroxides, thereby showing higher electrocatalytic activity and stability.

(2) The catalyst of the present invention can be directly used as an electrode material for electrocatalytic reaction. The nickel metal material and the copper material substrate can form a tightly combined heterojunction without additional binders, and the surface treatment is simple and gentle, which greatly improves the electrical conductivity. The structural stability of the catalytic electrode.

(3) The catalyst of the present invention can be assembled and applied to an electrolyzed water hydrogen production device, works well and stably, and has a large industrial application potential.

Description of the drawings

Figure 1 is a schematic diagram of the structure of the catalyst of the present invention,

Among them, 1-copper material, 2-cubic phase structure metallic nickel, 3-hexagonal phase structure metallic nickel, 4-nickel oxide or nickel hydroxide.

2 is a high-resolution scanning transmission microscope image and a Fourier transform image of the catalyst 1 of Example 1.

FIG. 3 is a Raman spectrum of Catalyst 2 of Example 2. FIG.

FIG. 4 is an X-ray photoelectron spectrogram of the catalyst 2 of Example 2. FIG.

FIG. 5 is a scanning electron micrograph of the catalyst 4 of Example 4. FIG.

Figure 6 is a polarization curve (LSV) diagram of the catalyst 1 of Example 1 in a 1M KOH electrolyte.

Among them, 1-catalyst 1, 2-platinum-carbon catalyst in comparative example 2, 3-comparative catalyst 1, 4-foam nickel, and 5-foam copper in comparative example 1.

Figure 7 is a long-term hydrogen evolution stability test diagram of catalyst 1 of Example 1 in 1M KOH electrolyte.

Among them, 1-Catalyst 1, 2-Platinum-carbon catalyst in Comparative Example 2.

Figure 8 is a long-term hydrogen evolution stability test diagram of the catalyst 2 of Example 2 in a 4M KOH electrolyte.

Among them, 1-catalyst 2 is used directly after preparation, 2-catalyst 2 is prepared and placed in air for 2 days, 3-catalyst 2 is prepared and placed in air for 4 days, 4-catalyst 2 is prepared and placed in air 6 days.

Figure 9 is a polarization curve (LSV) diagram of the catalyst 2 of Example 2 in a 4M KOH electrolyte.

Among them, 1-catalyst 2 is tested directly after preparation, and 2-catalyst 2 is tested after being tested in a harsh electrocatalytic environment with long-term, high-current, and high-alkaline concentration electrolyte.

Fig. 10 is a graph of the polarization curve (LSV) of the catalyst 6 of Example 6 in a 6M KOH electrolyte at 80°C.

FIG. 11 is a test diagram of the water splitting performance of the catalyst 1 in Example 1. FIG.

Fig. 12 is a diagram of a complete water dissolution device for catalyst 1 in Example 1.

Among them, a- the test diagram of the device, b- the appearance diagram of the device, c- the electrolytic cell diagram of the device, d- the electrolytic cell diagram of the device, e- the catalysts of different sizes 1

Detailed ways

The following specific examples illustrate the implementation of the present invention. Those familiar with the art can easily understand the other advantages and effects of the present invention from the content disclosed in this specification.

It should be noted that the structure, ratio, size, etc. shown in the accompanying drawings in this specification are only used to match the content disclosed in the specification for the understanding and reading of those who are familiar with the art, and are not intended to limit the implementation of the present invention. Therefore, it does not have technical significance. Any structural modification, proportional relationship change or size adjustment shall fall within the present invention without affecting the effects that can be produced by the present invention and the objectives that can be achieved. The disclosed technical content must be within the scope of coverage. At the same time, the terms "upper", "inner", "outer", "bottom", "one", "in" and other terms quoted in this manual are only for the convenience of description and are not meant to limit the present invention. The scope of implementation, and the change or adjustment of the relative relationship, shall be regarded as the scope of implementation of the present invention without substantial changes to the technical content, and shall be described first.

Example 1

Cut the foamed copper into small pieces of 1×2×2cm, degrease and remove oil in acetone ultrasonically for 10 minutes, rinse with ultrapure water for 3 times, put it in 1mol/L hydrochloric acid and ultrasonic for 10 minutes to remove surface oxides, and then rinse with ultrapure water 3 Second, obtain pre-treated foamed copper;

Weigh 0.33g of sodium formate and add it to 1.5ml of ultrapure water to obtain a sodium formate solution; Weigh 0.05g of NiCl ₂ ·6H ₂ O into 13ml of DMF to dissolve ultrasonically to obtain a nickel chloride solution;

Mix the sodium formate solution and the nickel chloride solution into a 25ml reactor after uniformly sonicating, add the pre-treated foamed copper, seal, and place in a temperature-programmed oven with a heating rate of 3°C/min and react at 160°C for 18 hours. Cooled to room temperature, took out the foamed copper, washed with water and ethanol alternately 3 times, dried in vacuum at 60°C for 12 hours, and then subjected to electrochemical hydrogen evolution polarization curve test, placed in the air at room temperature for 10 hours to oxidize to obtain catalyst 1.

Example 2

Weigh 0.2g of sodium formate and add it to 1.5ml of ultrapure water to obtain a sodium formate solution; weigh 0.2g of NiCl ₂ ·6H ₂ O into 13ml of DMF to dissolve ultrasonically to obtain a nickel chloride solution;

Mix the sodium formate solution and the nickel chloride solution into a 25ml reaction kettle after uniformly sonicating, add the pre-treated foamed copper in Example 1, seal it, and place it in a temperature-programmed oven at a heating rate of 3°C/min at 150°C React for 24 hours, cool to room temperature, take out the foamed copper, wash alternately with water and ethanol for 3 times, dry in vacuum at 60°C for 12 hours, and test the polarization curve of electrochemical hydrogen evolution, then place it in the air at room temperature for 6 hours to oxidize. Catalyst 2.

Example 3

Weigh 0.1g of sodium formate into 1.5ml of ultrapure water to obtain a sodium formate solution; weigh 0.4g of nickel acetylacetonate into 13ml of DMF to dissolve ultrasonically to obtain a nickel acetylacetonate solution;

Mix the sodium formate solution and the nickel acetylacetonate solution into a 25ml reactor after uniformly sonicating, add the pretreated foamed copper in Example 1, seal it, and place it in a temperature-programmed oven at a heating rate of 3°C/min at 160°C React for 20 hours, cool to room temperature, take out the foamed copper, wash alternately with water and ethanol 3 times, dry in vacuum at 60°C for 12 hours, and test the polarization curve of electrochemical hydrogen evolution, then place it in the air at room temperature for 10 hours to oxidize. Catalyst 3.

Example 4

Weigh 0.15g of sodium formate and add it to 1.5ml of ultrapure water to obtain a sodium formate solution; Weigh 0.7g of nickel nitrate and add 13ml of DMF to ultrasonically dissolve to obtain a nickel nitrate solution;

Mix the sodium formate solution and the nickel nitrate solution into a 25ml reactor after uniformly sonicating, add the pre-treated foamed copper in Example 1, seal it, and place it in a temperature-programmed oven with a heating rate of 3°C/min at 160°C React for 20 hours, cool to room temperature, take out the foamed copper, wash alternately with water and ethanol 3 times, dry in vacuum at 60°C for 12 hours, and then conduct electrochemical hydrogen evolution polarization curve test, then place it in the air at room temperature for 15 hours to oxidize to obtain a catalyst 4.

Example 5

The copper powder was ultrasonically degreasing and degreasing in acetone for 10 minutes, rinsed with ultrapure water for 3 times, placed in 2mol/L hydrochloric acid for 10 minutes to remove surface oxides, and then washed with ultrapure water for 3 times to obtain pretreated copper powder;

Weigh 0.06 g of sodium formate and add it to 1.5 ml of ultrapure water to obtain a sodium formate solution; weigh 0.1 g of nickel acetylacetonate into 13 ml of DMF to dissolve ultrasonically to obtain a nickel acetylacetonate solution;

Mix the sodium formate solution and the nickel acetylacetonate solution into a 25ml reactor after uniformly sonicating, add the pretreated copper powder, seal it, and place it in a temperature-programmed oven with a heating rate of 4°C/min and react at 180°C for 12 hours. After cooling to room temperature, the copper powder was taken out, washed with water and ethanol alternately 3 times, dried in vacuum at 60°C for 10 hours, and then subjected to the electrochemical hydrogen evolution polarization curve test, placed in the air at room temperature for 20 hours to oxidize, and catalyst 5 was obtained.

Example 6

The copper mesh is cut into small pieces of 0.5×3cm, degreasing and degreasing in acetone ultrasonically for 10 minutes, rinsed with ultrapure water for 3 times, placed in 1mol/L hydrochloric acid and ultrasonicated for 10 minutes to remove surface oxides, and then cleaned with ultrapure water for 3 times to obtain Pretreatment of copper mesh;

Weigh 0.25g of sodium formate and add it to 1.5ml of ultrapure water to obtain a sodium formate solution; weigh 0.3g of nickel nitrate into 13ml of DMF to dissolve ultrasonically to obtain a nickel nitrate solution;

Mix the sodium formate solution and the nickel nitrate solution into a 25ml reactor after uniformly sonicating, add a pre-treated copper mesh, seal, and place in a temperature-programmed oven with a heating rate of 3°C/min, react at 170°C for 15 hours, and cool At room temperature, take out the copper mesh, wash with water and ethanol alternately 3 times, dry at 50°C under vacuum for 20 hours, and test the polarization curve of electrochemical hydrogen evolution, place it in the air at room temperature for 5 hours to oxidize to obtain catalyst 6.

Example 7

The copper foil is cut into small pieces of 0.5×3cm, degreasing and degreasing in acetone ultrasonically for 10 minutes, rinsed with ultrapure water for 3 times, placed in 1mol/L hydrochloric acid and ultrasonicated for 10 minutes to remove surface oxides, and then cleaned with ultrapure water for 3 times to obtain Pretreatment of copper foil;

Weigh 0.12g of sodium formate and add it to 1.5ml of ultrapure water to obtain a sodium formate solution; Weigh 0.25g of nickel nitrate and add 13ml of DMF to ultrasonically dissolve to obtain a nickel nitrate solution;

Mix the sodium formate solution and the nickel nitrate solution into a 25ml reactor after uniformly sonicating and shaking, add the pretreated copper foil, seal, and place in a temperature-programmed oven at a heating rate of 3°C/min, react at 170°C for 15 hours, and cool. At room temperature, take out the copper foil, wash it with water and ethanol alternately 3 times, and dry it in vacuum at 50°C for 20 hours. After electrochemical hydrogen evolution polarization curve test, it is placed in the air at room temperature for 12 hours to oxidize, and catalyst 7 is obtained.

Comparative example 1

In the preparation step of Example 1, vacuum drying was performed at 60° C. for 12 hours without air oxidation, and comparative catalyst 1 was obtained.

Comparative example 2

Platinum-carbon catalyst supported on foamed copper substrate.

Electrocatalytic water splitting hydrogen production activity and stability test 1

The catalyst 1 prepared in Example 1 was subjected to the electrocatalytic water splitting hydrogen production test: a three-electrode test system was used on the electrochemical workstation CHI660E, the working electrode was the catalyst 1 electrode, the counter electrode was a graphite carbon sheet, and the reference electrode was mercury- Mercury oxide electrode. The test electrolyte is a 1mol/L potassium hydroxide aqueous solution at 25°C, and the test is saturated with high-purity nitrogen, and the test temperature is room temperature. In linear sweep voltammetry curve test, the sweep rate is 1mV/s, and the solution ohmic drop iR compensation correction is performed, and it is converted to the electrode potential of the reversible hydrogen electrode (RHE). The results of the stability test are recorded through the potential-time curve.

Figure 6 shows the polarization curve (LSV) of catalyst 1 in 1M KOH electrolyte. It can be seen that the electrode of catalyst 1 exhibits high hydrogen evolution activity in alkaline medium, and the current density is -10mA·cm ^-2 At and -100 mA·cm ^-2 , the overpotentials of the comparative catalyst 1 were 87mV and 157mV, respectively, while the overpotentials of the catalyst 1 of the present invention were only 54mV and 112mV, respectively. The activity of the catalyst prepared by the invention is significantly higher than that of blank nickel foam and copper foam, and is even similar to commercial precious metal platinum carbon electrodes under high current density. Fig. 7 is a long-term hydrogen evolution stability test diagram of catalyst 1 of the present invention in 1M KOH electrolyte. It can be seen that after ^{20 hours of continuous electrolysis at a current density of 20 mA·cm -2} , the hydrogen evolution overpotential of catalyst 1 is maintained at 73 mV There is no obvious attenuation on the left and right, while the platinum-carbon catalyst in Comparative Example 2 has a more obvious attenuation.

Electrocatalytic water splitting hydrogen production activity and stability test 2

The catalyst 2 prepared in Example 2 was used for the electrocatalytic water splitting hydrogen production test: according to the steps of the above electrocatalytic water splitting hydrogen production activity and stability test 1, the working electrode was changed to the catalyst 2 electrode, and the electrolyte was changed to 4mol/ L potassium hydroxide aqueous solution, the rest of the steps remain unchanged.

Figure 8 is a long-term hydrogen evolution stability test diagram of catalyst 2 in 4M KOH electrolyte. It shows that catalyst 2 can maintain stable activity at a higher current density and simulate the intermittent and voltage fluctuations during actual application. After testing, the catalyst 2 can still maintain good electrocatalytic stability after being placed in the air for 2 days, 4 days, and 6 days after the test. Figure 9 shows the polarization curve (LSV) of catalyst 2 tested under 4M KOH electrolyte at room temperature. It can be seen that catalyst 2 has been tested in a harsh electrocatalytic environment with a long time, high current, and high alkaline concentration electrolyte. After that, the catalytic activity can still be maintained well without decay. Therefore, the catalyst electrode of the present invention has good working tolerance of strong alkaline media.

Electrocatalytic water splitting hydrogen production activity and stability test 3

The catalyst 6 prepared in Example 6 was used for the electrocatalytic water splitting hydrogen production test: according to the steps of the above electrocatalytic water splitting hydrogen production activity and stability test 1, the working electrode was changed to the catalyst 3 electrode, and the electrolyte was changed to 80°C 6mol/L potassium hydroxide aqueous solution, the rest of the steps remain unchanged.

Fig. 10 is the polarization curve (LSV) diagram of the catalyst 6 of Example 6 in a 6M KOH electrolyte at 80°C. It can be seen that the electrode of the catalyst 6 can still perform higher in a strong and harsh alkaline medium. The hydrogen evolution activity.

Water decomposition performance test

The nickel nickel disulfide coated foamed nickel catalyst is used as a catalyst for complete water desorption and oxygen evolution. Catalyst 1 in Example 1 is used as a hydrogen evolution catalyst. It is assembled as a complete water desorption catalyst and installed in a self-designed large-area complete dewatering device for water decomposition. Performance Testing. Test method: The two-electrode test system is used on the electrochemical workstation CHI660E, the test electrolyte is 25℃ 1mol/L potassium hydroxide aqueous solution, the test temperature is room temperature, and the scan rate is 2mV/s during linear sweep voltammetry curve test.

Fig. 11 is a test chart of the water decomposition performance of the test. The ^{overpotential required for complete water dissolution of 10 mA·cm -2} is only 1.53V, and there is basically no attenuation of stable operation for 15 hours at a current density of ^{20 mA·cm -2.}

Attached Figure 12 is a diagram of the complete water solution device for this test.

Claims

A catalyst, characterized in that the substrate of the catalyst is a copper material, the surface of the copper material is coated with metallic nickel with a cubic phase structure, and the surface of the cubic phase structure metallic nickel is coated with a hexagonal phase structure metallic nickel. The surface of the structured metal nickel is provided with nickel oxide or nickel hydroxide; the thickness of the cubic-phase structured metal nickel is 10-200nm, the thickness of the hexagonal-phase structured metal nickel is 3-14nm, and the nickel oxide or nickel hydroxide The thickness is 1 to 5 nm.
The catalyst according to claim 1, wherein the thickness of the cubic phase structure metallic nickel is 30-180 nm.
The catalyst according to claim 1, wherein the thickness of the hexagonal phase structure metallic nickel is 4-12 nm.
The catalyst according to claim 1, wherein the thickness of the nickel oxide or nickel hydroxide is 2 to 4 nm.
The catalyst according to any one of claims 1 to 4, wherein the copper material is selected from at least one of foamed copper, copper powder, copper mesh and copper foil.
A method for preparing the catalyst according to any one of claims 1 to 5, characterized in that it comprises the following steps:

(1) The copper material is ultrasonically degreased and degreasing in acetone or absolute ethanol, washed with ultrapure water, placed in hydrochloric acid to ultrasonically remove surface oxides, and then cleaned with ultrapure water to obtain a pretreated copper material;

(2) In parts by weight, 1 part of sodium formate is added to 4 to 200 parts of water to obtain a sodium formate solution;

(3) In terms of parts by weight, 1 part of nickel source is added to 15-600 parts of N,N-dimethylformamide to obtain a nickel source solution;

(4) Add the pretreated copper material obtained in step (1), the sodium formate solution obtained in step (2), and the nickel source solution obtained in step (3) into the reaction kettle, seal, and react at 150-180°C After 12-24 hours, cool, take out the copper material, clean, vacuum dry, perform electrochemical hydrogen evolution polarization curve test, take it out, and place it in an oxygen atmosphere at room temperature for 5-20 hours to obtain the catalyst.
The preparation method according to claim 6, wherein the nickel source in step (3) is selected from at least one of nickel chloride, nickel sulfate, nickel nitrate and nickel acetylacetonate.
The preparation method according to claim 6, characterized in that, after copper material, sodium formate solution and nickel source solution are added to the reaction kettle in step (4), the reaction filling ratio is 45-75%.
The preparation method according to claim 6, wherein the oxygen atmosphere in step (4) is selected from oxygen or a mixed gas containing oxygen.
8. The preparation method of claim 6, wherein the mixed gas is air.
An electrocatalytic decomposition of water to produce hydrogen process, characterized in that the catalyst of the process uses the catalyst described in any one of claims 1-5.